Crystal growth from a melt contained in a crucible is one of the oldest established methods of crystal growth. The basic requirements of the method are very simple, but the presence of the crucible presents manufacturing difficulties which are not easily overcome. The process of growth through solidification of a melt can be divided into two classes based upon whether the crucible containing the melt moves relative to the temperature gradient or the temperature gradient is moved through a stationery crucible containing the melt. A large number of different metals and alkali halides crystals have been formed by these methods. Semiconductor materials such as germanium and galium arsenide crystals have been formed by this method. The details of such methods may be found in S. L. Zerfos, et al, "Crystal Growth at High Temperatures", Transaction Faraday Society, 1949, and J. C. Brice, "The Growth of Crystal from the Melt", published by John Wiley & Sons, Inc., New York, 1965, pages 121 through 134.
As indicated, many materials have been successfully grown in the directional solidification method. However, silicon has been a material that has presented great difficulty in growing crystals by this method. The primary problem with silicon is the cracking of the crucible or container walls upon reduction of the material to room temperature. Publications indicating this fact that silicon has not been successfully grown by this simple process are the W. R. Runyan "Silicon Semiconductor Technology" published by McGraw Hill Book Company, New York, 1965, and the W. R. Runyan, et al, U.S. Pat. No. 3,093,456.
B. Authier, "Novel Silicon Crystals and Method for Their Preparation", German Pat. No. P2508803.3-43, Sept. 9, 1976, is directed to an invention to form silicon by a casting and solidification technique to overcome the cracking problems of the prior art. This process involves pouring a silicon melt into a suitable mold, allowing the melt to solidify in a temperature gradient, which process is characterized be the fact that after the silicon melt is poured into the suitably shaped mold at least one of the two mutually opposing boundary surfaces of the melt is in contact with one surface of the mold and one surface in contact with the melt has a temperature of at most 1200.degree. C. The opposite boundary surface of the melt is subjected to a temperature at least 200 to 1000.degree. greater, but below the melting point of silicon. The purpose of the temperature gradient is to avoid the wetting of the crucible with the silicon and thereby preventing thermal strains when the material is cooled to room temperature.
It is also known that graphite is a durable substrate in contact with liquid silicon provided that the density of the graphite is greater than about 1.75 grams per cubic centimeter and the grain size is less than about 50 microns as indicated in T. F. Ciszek's Materials Research Bulletin No. 7, pages 731 through 738, 1972. The degree of carbon contamination of the silicon is small in the order of 20 parts per million, and similar to the level of oxygen contamination when silicon is grown from conventional silicon dioxide crucibles. However, unlike oxygen, carbon is not electrically active in silicon.
A portion of the present invention is disclosed in "Silicon Ribbon Growth by a Capillary Action Shaping Technique", by G. H. Schwuttke, et al., Annual Report, Oct. 1, 1977, JPL Contract: 954144, Subcontract under NASA Contract NAS7-100, pages 5-13.